G. N. Callahan

Bio: G. N. Callahan is an academic researcher from Colorado State University. The author has contributed to research in topics: Antigen & Major histocompatibility complex. The author has an hindex of 16, co-authored 39 publications receiving 1852 citations. Previous affiliations of G. N. Callahan include University of Florida & Scripps Health.

On the thymus in the differentiation of "H-2 self-recognition" by T cells: evidence for dual recognition?

Abstract: In the thymus, precursor T cells differentiate recognition structures for self that are specific for the H-2K, D, and I markers expressed by the thymic epithelium. Thus recognition of self-H-2 differentiates independently of the T cells H-2 type and independently of recognition of nonself antigen X. This is readily compatible with dual recognition by T cells but does not formally exclude a single recognition model. These conclusions derive from experiments with bone marrow and thymic chimeras. Irradiated mice reconstituted with bone marrow to form chimeras of (A X B)F1 leads to A type generate virus-specific cytotoxic T cells for infected targets A only. Therefore, the H-2 type of the host determines the H-2-restricted activity of killer T cells alone. In contrast, chimeras made by reconstituting irradiated A mice with adult spleen cells of (A X B)F1 origin generate virus-specific cytotoxic activity for infected A and B targets, suggesting that mature T cells do not change their self-specificity readily. (A X B)F1 leads to (A X C)F1 and (KAIA/DC) leads to (KAIA/DB) irradiation bone marrow chimeras responded against infected A but not B or C targets. This suggests that cytotoxicity is not generated against DC because it is abscent from the host's thymus epithelium and not against DB because it is not expressed by the reconstituting lymphoreticular system. (KBIB/DA) leads to (KCIC/DA) K, I incompatible, or completely H-2 incompatible A leads to B chimeras fail to generate any measurable virus specific cytotoxicity, indicating the necessity for I-specific helper T cells for the generation of killer T cells. Finally adult thymectomized, irradiated and bone marrow reconstituted (A X B)F1 mice, transplanted with an irradiated thymus of A origin, generate virus-specific cytotoxic T cells specific for infected A targets but not for B targets; this result formally demonstrates the crucial role of thymic epithelial cells in the differentiation of anti-self-H-2 specificities of T cells.

The lymphoreticular system in triggering virus plus self-specific cytotoxic T cells: evidence for T help.

Abstract: The thymus determines the spectrum of the receptor specificities of differentiating T cells for self-H-2; however, the phenotypic expression of T cell's specificity for self plus virus is determined predominantly by the H-2 type of the antigen presenting cells of the peripheral lymphoreticular system Furthermore, virus specific helper T cells are essential for the generation of virus-specific cytotoxic T cells For cooperation between mature T cells and other lymphocytes to be functional in chimeras, thymic epithelial cells and lymphohemopoietic stem cells must share the I region; killer T-cell generation also requires in addition compatibility for at least one K or D region These conclusions derive from the following experiments: A leads to (A X B)F1 chimeric lymphocytes do produce virus-specific cytotoxic T-cell activity for infected A but not for infected B cells; when sensitized in an acutely irradiated and infected recipient (A X B)F1 these chimeric lymphocytes respond to both infected A and B Therefore the predominantly immunogenically infected cells of chimeras the radiosensitive and by donor stem cells replaced lymphoreticular cells In this adoptive priming model (KAIA/DB leads to KAIA/DC) chimeric lymphocytes could be sensitized in irradiated and infected F1 against KA and DC but not against infected DB targets In contrast KBIB/DA leads to KCIC/DA chimeras' lymphocytes could not be sensitized at all in appropriately irradiated and infected F1 recipients Thus these latter chimeras probably lack functional I-specific T helper cells that are essential for the generation of T killer cells against infected D compatible targets If T cells learn in the thymus to recognize H-21 or K, D markers that are not at least partially carried themselves in other cells of the lymphoreticular system immunological interactions will be impossible and this paradox situation results in phenotypic immune incompetence in vivo

Restriction specificities, alloreactivity, and allotolerance expressed by T cells from nude mice reconstituted with H-2-compatible or -incompatible thymus grafts.

Abstract: Congenitally thymusless nude mice that lacked functional T cells were reconstituted with H-2-compatible or -incompatible thymus grafts taken from either fetal, newborn, or adult mice and transplanted under the kidney capsule or subcutaneously. Transplantation with unirradiated fetal (15--17 d) or newborn thymus grafts reconstituted the nude mice as assessed by their subsequent generation of virus-specific cytotoxic T cells in vivo or alloreactive T cells in vitro. The restriction specificity of T cells from homozygous mice was exclusively for the nude host H-2, as shown by direct cytolysis or by cold target competitive inhibition assays. irrespective of whether nude mice were reconstituted with H-2-compatible, semiallogeneic, or H-2-incompatible, unirradiated newborn or fetal thymus grafts (in order of decreasing efficiency of reconstitution). The restriction specificity for the nonhost H-2 of the thymus could not be demonstrated even after primary or secondary sensitization in an infected appropriate F1 environment. These nude mice reconstituted with fetal or newborn grafts were tolerant to the H-2 of the thymus donors. Nude mice transplanted with irradiated adult thymus grafts were reconstituted functionally with syngeneic or semisyngeneic but not with allogeneic thymus grafts. In homozygous nu/nu irradiated heterozygous recipients of F1 thymus grafts, the restriction specificity for the nonhost thymic H-2 could not be elicited upon adoptive sensitization in irradiated and infected F1 heterozygote stimulator mice; in fact, these chimeras' lymphocytes were not tolerant to the nonhost H-2. The discrepancy between the restorative capacity of unirradiated vs. irradiated thymus grafts suggests that precursors of T cells in nude mice can acquire restriction specificity and immunocompetence independently of a conventional, functioning H-2-compatible thymus if exposed to an allogeneic fetal or a newborn thymus that contains functioning thymocytes of donor type but not if reconstituted with an irradiated adult allogeneic thymus.

Biologic and Chemical Characterization of HLA Antigens in Human Serum

Abstract: HLA antigens of both the A and B loci were shown to be associated with the high density lipoprotein fraction of serum prepared by ultracentrifugal flotation. HLA-A9 antigens were purified 100-fold with essentially complete recovery by a simple procedure of high density lipoprotein preparation involving precipitation with polyanions and ultracentrifugal flotation. The purified lipid-associated antigen was immunogenic since it elicited the formation of cytotoxic xenoantibodies in rabbits. Serum HLA-A9 antigens were found by immunoprecipitation and gel electrophoresis to consist of a 45,000 m.w. heavy chain associated with β2-microglobulin. The size of the HLA-lipid complex (<190,000 m.w.) and of the HLA-deoxycholate complex (<102,000 m.w.) suggests that HLA antigens are shed into plasma as a complex of a single HLA molecule and a single β2-microglobulin chain, associated with boundary lipid.

Enhancement of antitumor immunity by CTLA-4 blockade.

Abstract: One reason for the poor immunogenicity of many tumors may be that they cannot provide signals for CD28-mediated costimulation necessary to fully activate T cells. It has recently become apparent that CTLA-4, a second counterreceptor for the B7 family of costimulatory molecules, is a negative regulator of T cell activation. Here, in vivo administration of antibodies to CTLA-4 resulted in the rejection of tumors, including preestablished tumors. Furthermore, this rejection resulted in immunity to a secondary exposure to tumor cells. These results suggest that blockade of the inhibitory effects of CTLA-4 can allow for, and potentiate, effective immune responses against tumor cells.

B lymphocytes secrete antigen-presenting vesicles.

Abstract: Antigen-presenting cells contain a specialized late endocytic compartment, MIIC (major histocompatibility complex [MHC] class II-enriched compartment), that harbors newly synthesized MHC class II molecules in transit to the plasma membrane. MIICs have a limiting membrane enclosing characteristic internal membrane vesicles. Both the limiting membrane and the internal vesicles contain MHC class II. In this study on B lymphoblastoid cells, we demonstrate by immunoelectron microscopy that the limiting membrane of MIICs can fuse directly with the plasma membrane, resulting in release from the cells of internal MHC class II-containing vesicles. These secreted vesicles, named exosomes, were isolated from the cell culture media by differential centrifugation followed by flotation on sucrose density gradients. The overall surface protein composition of exosomes differed significantly from that of the plasma membrane. Exosome-bound MHC class II was in a compact, peptide-bound conformation. Metabolically labeled MHC class II was released into the extracellular medium with relatively slow kinetics, 10 +/- 4% in 24 h, indicating that direct fusion of MIICs with the plasma membrane is not the major pathway by which MHC class II reaches the plasma membrane. Exosomes derived from both human and murine B lymphocytes induced antigen-specific MHC class II-restricted T cell responses. These data suggest a role for exosomes in antigen presentation in vivo.

MHC-restricted cytotoxic T cells: studies on the biological role of polymorphic major transplantation antigens determining T-cell restriction-specificity, function, and responsiveness.

Abstract: Publisher Summary This chapter focuses on the important discovery that virus-specific cytotoxic T cells are dually specific for virus and for a self cell surface antigen encoded by the major histocompatibility complex (MHC). The initial work was carried out on the lymphocytic choriomeningitis virus system but it soon became evident that the same phenomenon applied to many other viruses. In addition, the same principle has been found to hold for other antigenic systems, such as trinitrophenyl coupled to cells, minor histocompatibility antigens, and the H-Y model. Graft rejection and the need for genetically homogeneous inbred mouse strains for cancer research led to the development of transplantation immunology and immunogenetics. The result is that the gene complex coding for major transplantation antigens is one of the better understood mammalian genetic regions. Cytotoxic T-cell specificity is comparable to serological specificity. Because quantification of specificity or cross-reactivity is difficult, and because of the technical limitations of these cytotoxic T-cell assays, results are interpreted with great reservation. MHC restriction reflects the fact that the effector function of T cells is determined by the kind of Self-H recognized together with the foreign antigen on cell surfaces: K and D are receptors for lytic signals, I determinants are receptors for cell differentiation signals that are delivered antigen-specifically by T cells.

The SCID-hu mouse: murine model for the analysis of human hematolymphoid differentiation and function.

Abstract: The study of human hematopoietic cells and the human immune system is hampered by the lack of a suitable experimental model. Experimental data are presented showing that human fetal liver hematopoietic cells, human fetal thymus, and human fetal lymph node support the differentiation of mature human T cells and B cells after engraftment into mice with genetically determined severe combined immunodeficiency. The resultant SCID-hu mice are found to have a transient wave of human CD4+ and CD8+ T cells and human IgG (immunoglobulin G) in the peripheral circulation. The functional status of the human immune system within this mouse model is not yet known.